Petroleum-based hydrocarbons, such as crude oil, can be separated into four fractions based on solubility in certain solvents: saturate, aromatic, resin, and asphaltene. Asphaltene is defined as a fraction which is not soluble in an n-alkane, particularly, n-heptane. The other fractions, which are soluble in n-alkane, are referred to as maltene.
There are many impurities in petroleum-based hydrocarbons, including, for example metals, sulfur, hydrogen, carbon, and components that include these impurities. Metals are primarily concentrated in the resin and asphalthene fractions; the remaining fractions can contain small amounts of metals. Vanadium, nickel and iron are the most frequently found metals in crude oil. In general, the asphalthene fraction has a higher concentration of vanadium than the resin fraction.
Metals found in petroleum-based hydrocarbons can cause severe problems in refining and other downstream processes such as petrochemical production processes. For example, metal compounds poison refining catalysts commonly used to enhance the processing of crude oil to meet the refined product specifications, for refining products such as gasoline and diesel. Metal compounds, particularly vanadium, in hydrocarbon-based liquid fuels can cause corrosion problems in hydrocarbon combustion processes, for example those used in power generation processes. In hydrocarbon combustion processes that employ gas turbines, the vanadium compound in the liquid fuel to the gas turbines can form vanadium oxide which can cause severe corrosion to metallic parts of the gas turbines.
Current methods of addressing the presence of metals in hydrocarbon-bearing petroleum streams include the use of additives injected with the hydrocarbon-bearing petroleum stream and processing steps to remove the metals before using the stream in a power generation process. In one application, additives are injected to trap vanadium compounds in a combustor. The additives suppress the corrosion effect of the vanadium compounds. While additives are effective to an extent, they cannot remove the metal compounds and therefore cannot completely prevent corrosion due to the presence of metals.
In conventional processing units, metal compounds are removed from the crude oil itself or from the its derivatives, such as refinery streams like residue streams. In a conventional hydroprocessing system, removal of metal compounds is achieved by a hydroprocessing unit where hydrogen is supplied in the presence of a catalyst. Metal compounds decompose through reactions with hydrogen and are then deposited on the catalyst. In most practices, following a period of operation the spent catalyst can be disposed. One of the disadvantages of conventional hydroprocessing systems involving catalysts is that it is nearly impossible to regenerate spent catalyst having deposited metals such as vanadium and nickel. Although conventional hydroprocessing can remove substantial amounts of metals from hydrocarbon streams, the process consumes huge amounts of hydrogen and catalyst. The short catalyst lifetime and huge hydrogen consumption contribute significantly to the costs associated with operating a hydroprocessing system. Large capital expenditures required to build a hydroprocessing unit coupled with the operating costs make it difficult for power generation plants to adopt such a complicated process as a pre-treatment unit of liquid fuel.
Another process that can be used to remove metals from petroleum-based hydrocarbons is a solvent extraction process. One such solvent extraction process is a solvent deasphalting (SDA) process. An SDA process can reject all or part of the asphalthenes present in a heavy residue to produce deasphalted oil (DAO). By rejecting the asphaltenes, the DAO has lower amount of metals than that of the feed heavy residue. The high removal of metals comes at the expense of liquid yield. For example, it is possible to reduce the metal content of an atmospheric residue from a crude oil from 129 part per million by weight (ppm by wt) to 3 ppm by wt in an SDA process; however the liquid yield of the demetallized stream is only around 75 volume percent (vol %).
Metals can be concentrated into certain parts of the petroleum products where the carbon to hydrogen ratio is higher than in other parts. For example, the coke or coke-like parts often contain highly concentrated metals. Specifically, vanadium can be concentrated into coke when heavy oil is treated with supercritical water under coking conditions, generally at high temperatures. Although coke formation could be beneficial to remove metals from liquid phase oil products, there are problems caused by coke: process lines are plugged by coke; liquid yield decreases with increasing amount of coke.
Supercritical water has unique properties which makes it suitable as a reaction medium for processing petroleum for certain reaction objectives such as upgrading and demetallization. Supercritical water is water above the critical temperature of water and above the critical pressure of water. The critical temperature of water is 373.946 degrees Celsius (° C.). The critical pressure of water is 22.06 megapascals (MPa). Supercritical water acting as a diluent prevents coke formation even without an external supply of hydrogen. The basic reaction mechanism of supercritical water mediated petroleum processes is the same as a radical reaction mechanism. Thermal energy creates radicals through chemical bond breakage. Supercritical water then creates a “cage effect” whereby radicals are surrounded by supercritical water and thus cannot react easily with each other. The cage effect enables supercritical water processes to have reduced coke formation as compared to conventional thermal cracking processes, such as delayed coker. “Coke” is generally defined to be the toluene insoluble material present in petroleum.
The majority of metals present in the resin and asphalthene fractions are known to be present as porphyrin-type compounds, where the metals are bonded to nitrogen by coordinative covalent bonds. The other forms of metal compounds have not been well identified, but at least some of the metal compounds exist as chelate type compounds.
A method that can remove metals from petroleum-based hydrocarbons while achieving high liquid yield is desired. A method that removes metals while reducing coke formation, minimizing generation of gas-phase product, and increasing liquid yield is desired.